Several recent studies have reported significant health effects of air pollution even at low levels of air pollutants. These studies have been criticized for the statistical methods and for inconsistency in results between cities. An important development in air pollution epidemiology has come from multicenter studies. Within the APHEA-2 project we have developed a statistical methodology to evaluate short-term health effects of air pollution using data from 30 cities across Europe. For the analysis, a hierarchical modelling approach was adopted and implemented in two stages: (a) data from each city were analyzed separately to allow for local differences, using generalized additive Poisson regression models; (b) city-specific effects estimates were regressed on city-specific covariates to obtain an overall estimate and to explore heterogeneity across cities. In order to illustrate our methodology we present results for PM10 effects. It was found that a 10 microg/m3 increase in PM10 or NO2 concentrations is associated with a 0.67% (95% CI: 0.50 to 0.90) and 0.33% (0.20 to 0.40) increase in total mortality, respectively. After mutual adjustment, the PM10 effect was reduced by 40% and that of NO2 by 20%, but both pooled estimates remained significant. Long-term mean NO2 concentrations act as an effect modifier for PM10 effects, even after adjustment for NO2 confounding effects. In the second stage we explored two different models for combining the adjusted for NO2, PM10 effects across cities: bivariate, which accounts for within-city correlation of PM10 and NO2; and univariate, which ignores this correlation. Both models gave broadly the same results